Democratic Nominee for President of the United States former Secretary of State Hillary Clinton

Melina Mara | The Washington Post | Getty Images

The behemoth super PAC backing Hillary Clinton is targeting two vulnerable Republican senators in the final push to Election Day.

Priorities USA Action reported spending nearly $500,000 this week on television ads opposing Sens. Pat Toomey of Pennsylvania and Kelly Ayotte of New Hampshire, according to a Federal Election Commission filing Wednesday. The PAC has shelled out about $700,000 total in those races.

Both incumbent senators are locked in tight contests that will help to determine which party controls the Senate. The Pennsylvania race has drawn huge cash from independent groups and is already the most expensive Senate contest ever. The New Hampshire race has seen the second-most money of this cycle's Senate contests, also driven mostly by the flow of independent money.

The pro-Clinton Priorities USA has drawn huge sums from mega donors like George Soros, Haim Saban and Donald Sussman as it does not face the contribution limits Clinton's campaign committee does. Priorities USA Action has broken super PAC fundraising records with nearly $175 million raised, The Washington Post reported this week.

The PAC had spent more than $130 million in this election cycle at the start of October, with most of that cash directed to the presidential race.

But it has started to direct some of its resources to Senate races as the Democrats attempt to take control of the chamber. The Democrats need four Senate seats to gain a majority if Clinton wins the presidency and five if she does not. Pennsylvania and New Hampshire will prove crucial in determining the balance of power.

Averages of recent surveys show both Toomey and Ayotte with roughly 1-point leads in their races, according to RealClearPolitics.